Magnesium is often formed into ingots, bars, sheets, or rods, and sold as a commercial product. Typically, the magnesium is given a particular shape by a casting process which involves melting the magnesium and pouring it into a mold.
Magnesium in the molten state, however, requires some form of protection to inhibit oxidation and to prevent burning. When molten magnesium is exposed to ambient air, a thick layer of magnesium oxide forms on the surface of the melt. However, this layer of magnesium oxide is not adequate to provide the necessary protection. In a matter of just seconds, many blooms form on the surface of the molten magnesium which first glow red and then burst into white flames. As a result, the MgO (on the surface) is not smooth or coherent with the molten surface. Moreover, MgO occupies much less volume than the metal from which it is formed. Thus, the entire surface of the molten magnesium that is exposed to ambient air is not protected. In addition, the boiling point of magnesium is relatively low. This results in a high vapor pressure which further puts critical demands on the MgO layer.
One solution to the oxidation problem has been to isolate the melt from the air with a non-reactive barrier consisting of a flux of salts (e.g., 50% MgCl.sub.2, 25% KCl, 18% BaCl.sub.2, 4% CaF.sub.2, 3% MgO). The flux is liquid at the temperature of molten magnesium and has a surface tension that enables the flux to encase the melt with a relatively impermeable fluid film.
However, the use of a salt flux cover has a number of drawbacks. For example, it reduces the magnesium metal yield. The salt flux forms a sludge composed of oxides and inter-metallic particles. The sludge is generated by oxidation and agitation of the molten metal bath. Oxidation of the salt flux also produces dross floating on the metal surface. Both the sludge and the dross have a considerable amount of metallic magnesium trapped in their structure which reduces yield.
The sludge and dross also create other problems. For example, the sludge settles to the bottom of the pot where it may insulate and cause hot spots on the pot. Additionally, both present disposal problems. Flux can contaminate (e.g., corrosion effects) the finished castings by forming flux inclusions or oxides. Flux fumes and flux dust can also cause serious corrosion problems in the plant.
Another approach to solving the problem associated with molten magnesium is to use inhibitors in the air. The early practice was to burn coke or sulfur to produce the gaseous agent, CO.sub.2 or SO.sub.2. An atmosphere of CO.sub.2 was found to be superior to the commonly used commercial atmospheres of N.sub.2, Ar, or He because of the absence of vaporization of the magnesium, the absence of excessive reaction products, and the necessity for the enclosure above the molten metal to be extremely air tight. Some of these gases are also very expensive to use. Additionally, small amounts of SO.sub.2 (&lt;1%) was found to be sufficient for protecting the molten magnesium from burning, even if air and moisture are present in the atmosphere above the melt.
However, the use of these inhibitors also has several drawbacks. For example, both CO.sub.2 and SO.sub.2 pose pollution problems such as breathing discomfort for plant personnel, residual sludge disposal, and a corrosive atmosphere which is detrimental to both plant and equipment. Furthermore, SO.sub.2 is toxic and can cause explosions.
While BF.sub.3 has been mentioned as being a very effective inhibitor, it is not suitable for commercial processes because it is extremely toxic and corrosive.
Sulfur hexafluoride (SF.sub.6) has also been mentioned as one of many fluorine-containing compounds that can be used in air as an oxidation inhibitor for molten magnesium. However, it was overlooked for many years. When its important dielectric properties were applied to electric equipment, it was tried as an inhibitor in die casting operations. SF.sub.6 was discarded at that point, however, because of its severe attack on the ferrous equipment. In addition, the use of pure SF.sub.6 for protecting molten magnesium has been reported to have caused explosions.
Later, it was found that at low concentrations of SF.sub.6 in air (&lt;1%), a protective thin film of MgO (and MgF.sub.2) is formed on the magnesium melt surface. Advantageously, even at high temperatures in air, SF.sub.6 showed negligible or no reactions.
However, the combination of SF.sub.6 and air has some drawbacks. For example, since SF.sub.6 is adsorbed by the thin film, there must be a continuous supply of the gas to maintain the otherwise unstable film. Moreover, at higher temperatures, higher concentrations of SF.sub.6 are necessary. At the higher concentrations, SO.sub.2 and MgF.sub.2 are formed, and magnesium is wastefully further oxidized.
It was then found that CO.sub.2 could be used together with SF.sub.6 and air. A gas atmosphere of air, SF.sub.6, and CO.sub.2 has several advantages. First, it is non-toxic and non-corrosive. Second, it eliminates the use of salt fluxes and the need to dispose of the resulting sludge. Third, using such a gas atmosphere results in a lower metal loss, elimination of corrosion effects, and clean castings. Fourth, such a casting process provides a clean operation and improves the working conditions. In short, the addition of CO.sub.2 provides much improved protection compared to the melt protection obtained with air/SF.sub.6 mixtures.
However, using a gas atmosphere of SF.sub.6 and CO.sub.2 also has some disadvantages. Namely, both SF.sub.6 and CO.sub.2 have been mentioned as being greenhouse gases, i.e., they have a high global warming potential. In addition, SF.sub.6 is a relatively expensive gas. Thus, there is a need in the art to reduce the total amount of SF.sub.6 and CO.sub.2 that is used and released into the atmosphere.
Accordingly, it is an object of the present invention to address this need in the magnesium industry. In particular, it is an object of the present invention to provide an efficient and economical process for recovering SF.sub.6 and/or CO.sub.2 from the vent gas of a magnesium foundry and optionally recycling the recovered gas. These and other objects of the invention will become apparent in light of the following specification, and the appended drawings and claims.